1. Field of the Invention
This invention relates to a virtual network system in which data communication is carried out among a plurality of work stations connected to LAN segments, and more particularly to a virtual network system for performing efficient data communication between a plurality of work stations by emulating existing LAN on an ATM network.
2. Description of the Related Art
In the prior art, systems are known that have a plurality of computers connected together with a communication cable to construct a LAN (Local Area Network) or to connect a plurality of LANs having the aforesaid plurality of computers connected thereto through a communication line to construct a WAN (Wide Area Network). For the LAN and WAN systems, data communication is provided for transmitting or receiving data between the computers connected thereto.
As one example of the aforesaid prior art network, there is provided a network for performing data communication by connecting a plurality of computers with a communication cable known as Ethernet and for transmitting or receiving a MAC (Media Access Control) frame on the Ethernet.
Data is transmitted from a computer of a transmitting side to another computer of a receiving side using a communication protocol such as TCP/IP (Transmission Control Protocol/Internet Protocol) in the prior art Ethernet network. For example, at first, the transmitting side computer adds information such as the IP (Internet Protocol) address to make an IP packet. The IP address is a network address for recognizing the receiving side computer for the data being sent over the Ethernet. Then, the transmitting side computer connected to the Ethernet performs an address resolution for getting a MAC address of the receiving side computer from the IP address of the receiving side computer in accordance with another protocol, such as ARP (Address Resolution Protocol). Thereafter, the transmitting side computer adds information such as the aforesaid attained MAC address of the receiving side computer to the IP packet to make a MAC frame and then transmits the MAC frame onto the network.
Each computer connected to the prior art network using the Ethernet receives the MAC frame transmitted as described above, and determines whether or not it is the computer specified by the transmitted MAC address. In the case that the MAC address specified as the destination address is coincident with the MAC address of a particular computer, the computer reads out the IP packet within the MAC frame. In the case that the MAC address of the specified destination does not coincide with the MAC address of the computer, the computer discards the MAC frame. Thus, data communication between the transmitting side computer and the receiving side computer is performed.
The IP address used in the prior art Ethernet network is an address defined for the network layer (the third layer) of a 7-layer model of OSI (Open Systems Interconnection). The IP address is stored in a storage device of the computer by software executed when the computer starts to operate. Software, which uses the communication protocol such as TCP/IP or the like, uses the IP address to recognize each of the plurality of the computers connected to the network. The IP address is used when the computer is recognized on the network with the software used by the and further which can be changed into an optional value in response to a content of the software executed when the computer starts to operate.
In addition, the MAC address is an address defined for a data link layer (the second layer) of the OSI 7-layer model. The MAC address is a hardware address specific to a respective network interface card/chip or the like, and is assigned to the network interface card/chip which connects the computer to the network. The MAC address can not be changed.
In the prior art network using the Ethernet, the Ethernet acting as the communication medium is shared by a plurality of computers and one IP packet transmitted onto the Ethernet can be received by a plurality of computers, thereby either a broadcast or a multi-cast communication to the plurality of computers can be easily performed.
As another example of a prior art network, there is provided a network using ATM (Asynchronous Transfer Mode) and the network using the aforesaid ATM is already described in a publication such as "ATM-LAN" edited by Hiroshi Shimizu and Hiroshi Suzuki, Soft Research Center. In the ATM network, a plurality of computers are connected to an ATM switch and an an ATM cell of 53 bytes is transmitted/received through the ATM switch to perform the data communication.
In the case that the specific data is transmitted from the transmitting side computer to the receiving side computer in accordance with the communication protocols such as TCP/IP in the ATM network, for example, at first, the transmitting side computer adds information such as an IP address for making the receiving side computer recognize the transmitted data.
Then, the transmitting side computer performs an address resolution for getting the ATM address of the receiving side computer from the ATM address resolution server having an address administration table, which administrates the ATM addresses corresponding to the IP addresses of a plurality of computers connected to the ATM network. After getting the ATM address of the receiving side computer from the ATM address resolution server, the transmitting side computer constructs a connection with the receiving side computer by using the ATM address of the receiving side computer. After establishing a connection with the receiving side computer, the transmitting side computer constructs makes an ATM cell from the IP packet and transmits the produced ATM cell to the receiving side computer.
The receiving side computer, which the transmitting side computer has established a connection with, receives a plurality of ATM cells sent from the transmitting side computer and assembles the IP packet from the plurality of received ATM cells. Thus, data communication between the transmitting side computer and the receiving side computer is performed.
In the aforesaid ATM network, ATM cells may be made from a packet using an upper level protocol higher than or equal to the network layer of the existing network, which is transmitted and received on the ATM network and assembled into the packet. This manner for transmitting and receiving packets of an upper level protocol higher than or equal to the network layer is known as native-mode ATM. In the case that the packet is an IP packet, this is known as IP-over-ATM.
It is well known that the ATM network has superior functions such as setting a scalable transmitting speed or communication quality for every receiver as compared with the prior art network using the aforesaid Ethernet. However, a so called face-to-face communication after establishing the connection in the aforesaid ATM network is fundamental to its operation, so that broadcast or multi-cast communication may not be performed without applying special techniques.
In the case that the aforesaid ATM network is introduced to a new or existing network system, there are a lot of needs to operate the network system with an ATM network connected to the existing Ethernet, or to run the existing application software, which is made under an assumption of using the Ethernet, on the computer connected to the ATM network, because the network system using the aforesaid Ethernet and the like has already been distributed broadly.
As described above, in the case that the ATM network is connected to an existing LAN network or the existing application software operates on a computer connected to the ATM network, a LAN emulation for emulating the MAC protocol using the existing network to transmit/receive the MAC frame is carried out on the ATM network.
For example, in the case that the transmitting side computer transmits the specified data to the receiving side computer through the application software using the existing Ethernet in the ATM network, at first, the transmitting side computer adds information such as an IP address or the like for the receiving side computer to the aforesaid specific data to make the IP packet, thereafter adds information such as the MAC address of the receiving side computer to the aforesaid IP packet to make the MAC frame.
Then, the transmitting side computer inquires of the LAN emulation server the ATM address corresponding to the MAC address of the receiving side computer. The LAN emulation server, having an address administration table where the ATM addresses correspond to the MAC addresses of a plurality of computers connected to the network are registered, replies to the inquiry. Thus, the transmitting side computer performs an address resolution for getting the ATM address of the receiving side computer.
The transmitting side computer, which obtains the ATM address of the receiving side computer from the LAN emulation server, establishes a connection with the receiving side computer by using the ATM address of the receiving side computer, and makes ATM cells from the MAC frame and transmits them to the receiving side computer. The receiving side computer, having established a connection with the transmitting side computer, receives a plurality of ATM cells sent from the transmitting side computer, constructs the MAC frame from the plurality of received ATM cells, and thereafter reads out the IP packet from the constructed MAC frame. Thus, a data communication between the transmitting side computer and the receiving side computer is performed.
The LAN emulation is carried out on the ATM network as described above to enable the data communication using the ATM network without changing either the network interface card or the application software of the computer connected to the existing network.
In addition, since a BUS (Broadcast and Unknown Server) for performing either the broadcasting or a multi-casting is used in the ATM network for performing the LAN emulation, it is possible to perform either the broadcasting or the multi-casting on the ATM network in an easy manner.
In the case that a plurality of emulated LANs are set by grouping a plurality of computers performing the LAN emulation into a plurality of groups in the ATM network to construct a virtual LAN, ATM routers, each of which is a router having an ATM interface are used to connect the plurality of emulated LANs each other, therein so as to perform forwarding between the aforesaid plurality of emulated LANs.
The aforesaid ATM router receives a plurality of ATM cells sent from the computer within the specified emulated LAN to construct the MAC frame, thereafter reads out the IP address of the receiver (receiving side computer) stored in the IP packet within the constructed MAC frame and transmits the plurality of received ATM cells to the receiving side computer on the emulated LAN in the case that the read-out IP address of the receiver is the IP address of the computer connected to another emulated LAN.
In addition, in the case that the IP address read-out from the IP packet in the constructed MAC frame is not the IP address of the computer connected to another emulated LAN, the ATM router does not transmit the received ATM cells to the computer on another emulated LAN.
As described above, a plurality of emulated LANs are connected through the ATM router in the prior art ATM network, thereby an IP packet is relayed from an emulated LAN to another emulated LAN and at the same time IP packets caused by a broadcast or multi-cast communication within one emulated LAN is kept from being sent to any other emulated LANs.